No contingencies: Attentional prioritization by big or bright singletons

Goal-directed and stimulus-driven processes interact to control visual selective attention. However, there is much disagreement about whether stimulus-driven processes can lead to involuntary attentional capture. We conducted four experiments to assess attentional prioritization by stimuli that were singletons with respect to size or brightness (cf. Yantis and Egeth, 1999). One would predict that these features, which were unrelated to the target-defining features of the participant's task, would not receive attentional priority according to the contingent involuntary orienting hypothesis (Folk, Remington, & Johnston, 1992). We tested this hypothesis with a standard visual search for a vertical target bar among distractor bars tilted 30 degrees to the left or right of vertical. The singleton (a long bar or a bright bar) had a 1/n probability of being the target, with n the display size of 3, 6, or 9. Each participant only observed singletons of a single type in Experiment 1. In Experiment 2, each participant viewed both types, in mixed trials. In Experiment 3 the salience of the singletons was increased to test whether this translated into increased attentional prioritization. A neutral experiment without singletons was also conducted. The singletons consistently attracted attentional priority as the slope of the reaction time versus display size function was substantially shallowerwhen the target was the singleton than when it was not. However, they did not receive full attentional priority, as those slopes were significantly greater than zero, even with the very salient singletons used in Experiment 3. These results indicate that although abrupt onsets may be unique in receiving full attentional priority (i.e., near zero slope), brightness and size singletons do attract a significant degree of attentional priority under conditions where this would not be expected on the basis of the contingent involuntary orienting hypothesis.